Hundreds of species of commensal microorganisms are harbored in the gastrointestinal tracts of mammals, where they interact with the host immune system. Research using germ-free (GF) animals has shown that the commensal microorganisms influence the development of the mucosal immune system, such as histogenesis of Peyer's patches (PPs) and isolated lymphoid follicles (ILFs), secretion of antimicrobial peptides from the epithelium, and accumulation of unique lymphocytes in mucosal tissues, including immunoglobulin A-producing plasma cells, intraepithelial lymphocytes, IL-17-producing CD4-positive T cells (Th17), and IL-22-producing NK-like cells (Non-Patent Literature (NPL) 1 to 7). Consequently, the presence of intestinal bacteria enhances protective functions of the mucous membranes, enabling the host to mount robust immune responses against pathogenic microbes invading the body. On the other hand, the mucosal immune system maintains unresponsiveness to dietary antigens and harmless microbes (NPL Document 3). Abnormality in the regulation of cross-talk between commensal bacteria and the immune system (intestinal dysbiosis) may lead to overly robust immune response to environmental antigens and inflammatory bowel disease (IBD) may result (NPL 8 to 10).
Recent studies have shown that individual commensal bacteria control differentiation of their specific immune cells in the mucosal immune system. For example, Bacteroides fragilis, which is a commensal bacterium in humans, specifically induces a systemic Th1 cell response and a mucosal IL-10-producing T cell response in mice, and plays a role in protecting the host from colitis, which is caused by a pathogen (NPL 3). Segmented filamentous bacteria, which are intestinal commensal bacteria in mice, induce mucosal Th17 cell response and enhance resistance against infection of gastrointestinal tracts of the host with a pathogen (NPL 11 to 13). In addition, short-chain fatty acids derived from several commensal bacteria are known to suppress intestinal inflammation (NPL 14). Moreover, it has been observed that the presence of some species of intestinal microbiota greatly influences the differentiation of regulatory T cells (hereafter referred to as “Treg cells”) which help maintain homeostasis of the immune system. Although specific species of murine bacterial commensals that can strongly stimulate Tregs have been identified (NPL 15), it is still unknown whether species of human commensal bacteria exert an equivalent influence on the human immune system. Furthermore, the human intestinal tract harbors more than a thousand bacterial species, many of which have not yet been cultured (NPL 16). It is not feasible to guess a priori which ones, if any, might have an effect on Tregs.
In order to develop drugs, dietary supplements, or foods with beneficial immune functions for human use, it is desirable to identify commensal microorganisms that naturally colonize humans and have immune-modulating properties. Furthermore, since many of the commensals in the human microbiome have yet to be cultured, it is necessary to develop methods to cultivate them so that they can be produced by traditional industrial fermentation processes and subsequently incorporated in pharmaceutical or food formulations.
CD4+ T cells are regulatory T cells that have been identified as a cell subset that suppresses immunity. A transcription factor, Foxp3, is expressed in CD4+ T cells, which are known to play an important role in maintaining immunological homeostasis (NPL 8, 9, 17, and 18). Foxp3-expressing cells are present in large numbers in the colon and only Treg cells present locally in the colon constantly express IL-10, an immunosuppressive cytokine, at a high level (NPL 19). Animals having CD4+ Foxp3+ cells from which IL-10 is specifically removed develop inflammatory bowel disease (NPL 20).
Accordingly, there is a need to identify human-derived commensal bacterial species with the ability to strongly induce Treg cells to produce IL-10 in the colon at a high level and to develop methods to culture such species. Such species could be used to enhance immunosuppression, which, in turn, can be applied to treatment of autoimmune diseases, such as inflammatory bowel disease, inflammatory diseases, allergies, or organ transplantation, among other diseases and conditions.